* Get default for backend var defined in mixin
If the backend var is defined in a mixin class, it won't appear in
`cls.__dict__`, but the value is still retrievable via `getattr` on `cls`.
Prefer to use the actual defined default before using
`Var.get_default_value()`.
If `Var.get_default_value()` fails, set the default to `None` such that the
backend var still gets recognized as a backend var when it is used on `self`.
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Update test_component_state to include backend vars
Extra coverage for backend vars with and without defaults, defined in a
ComponentState/mixin class.
* fix integration test
5c0518053d